Chromamines And Cobaltamlnes

Description

Chromamines And Cobaltamlnes

Chromium hydroxide, when digested with excess of ammonia and ammonium chloride, forms a deep red solution; and on exposing it to air, a violet powder separates, of the formula CrCl3.4NH3.H2O. This powder, heated to ioo°, loses its water of crystallisation, and the residue has the formula C1CI3.4NH3. The ammonia is not expelled until the temperature 200° is reached. Salts containing chromium have been prepared, in which 3, 4, 5, 6, and 7 molecules of ammonia are associated with the original chromium salt. They find their explanation by a hypothesis like the one given, or the one on the following page.

Similar compounds are known with triad cobalt. On adding a solution of ammonia to cobalt sulphate, the precipitate at first formed (a basic sulphate) dissolves; exposure to air causes the oxidation of the cobalt from dyad to triad, and a black powder is deposited. On careful addition of hydrochloric acid, keeping the mixture cold, the colour of the powder changes to red; the compound has the formula CoCl3.4NH3.H2O, and is analogous to the chromium compound mentioned above. Other salts of this base have been made; they are termed roseo-cobaltamines. If temperature be allowed to rise, during the addition of hydrochloric acid to the oxidised solution of cobalt sulphate in ammonia, an isomeric substance is produced, containing no water of crystallisation, and having a purple colour. Other salts are known ; they are termed salts of purpureo-cobaltamine.

Such salts can be arranged in four classes, according to the number of electrons which they transfer to the anion on solution. These are, with cobalt: (1) non-ionised molecules, e.g. CoX3(NH3)3; (2) monovalent kations, e.g. {CoX2(NH3)4}X ; (3) divalent kations, e.g. {CoX(NH3)5}X2; and (4) trivalent kations, e.g. {Co(NH3)6}X3. In these formulae, the X within the bracket may be CI, Br, NO2, NO3, etc.; and H2O may partially replace NH3. Indeed, when the cobaltic group has the formula {(NO2)4Co(NH3)9}, it acquires acidic properties ; i.e. it withdraws an electron from a monovalent metal, such as sodium, forming Na{(NO2)4Co(NH3)2}. It is impossible, on the ordinary conceptions of valency, to account for the cobaltic group. Some of these compounds exhibit stereo-isomerism (see Vol. I. p. 104; also Stewart's " Stereo-Chemistry," pp. 270 et seq.). Chromium, iridium, and platinum form similar compounds.